ABS control which prevents wheels of a vehicle from slipping has been known. When the wheel speed decreases with respect to the vehicle body speed and a slip ratio, which is expressed as the quotient of the difference between the wheel speed and the vehicle body speed divided by the vehicle body speed, exceeds a predetermined threshold value, ABS control make the wheel speed to return a value closer the vehicle body speed and prevents the wheel from slipping by starting a pressure decrease in the wheel cylinder (hereinafter simply referred to as “W/C”) pressure.
With this kind of ABS control, when the coefficient of friction of the road surface on which the vehicle is traveling (hereinafter referred to as “road surface μ value”) is the same for all four wheels, a threshold value which becomes the ABS control start reference for each wheel is set to be the same. In a typical passenger vehicle, ABS control is performed independently for the left and right front wheels, and ABS control is performed simultaneously for the left and right rear wheels.
In cases such as when the road surface μ value for the left wheel is different from that for the right wheel, as it is on a split road surface, however, the threshold values for starting ABS control in the left and right wheels are different. Therefore, ABS control such as that described below is conventionally executed.
For example, when ABS control is performed independently for the left and right front wheels and ABS control is performed simultaneously for the left and right rear wheels, if a split road surface is detected, a select high control or a select low control is employed (see Japanese Patent Application Publication No. JP-A-08-099622, for example).
Select high control refers to starting a pressure decrease control simultaneously for the left and right wheels when the slip ratio of the wheel that is rotating faster, i.e., the wheel on the side with the road surface having a high road surface μ value (hereinafter simply referred to as “high μ road surface”), exceeds a threshold value for starting pressure decrease control in ABS control.
Select low control conversely refers to starting a pressure decrease control simultaneously for the left and right wheels when the slip ratio of the wheel that is rotating slower, i.e., the wheel on the side with the road surface having a low road surface μ value (hereinafter simply referred to as “low μ road surface”), exceeds a threshold value for starting pressure decrease control in ABS control.
In the control mode in which select high control is employed, the threshold value for starting pressure decrease control in ABS control in the wheel on the high μ road surface side is set on the low side and the start of pressure decrease control for the wheel on the low μ road surface side is delayed until the slip ratio of the wheel on the high μ road surface side exceeds the threshold value for starting pressure decrease control. Accordingly, braking force of the wheel of the high μ road surface side is inhibited from being reduced more than is necessary.
In the control mode in which select low control is employed, the wheel on the low μ road surface side is reliably prevented from slipping by also starting the pressure decrease control for the wheel on the high μ road surface side when the slip ratio of the wheel on the low μ road surface side exceeds the threshold value for starting pressure decrease control.
In this select low control, the threshold value to start pressure decrease control of the wheel on the low μ road surface side is set, but the threshold value to start pressure decrease control of the wheel on the high μ road surface side is not specifically set. That is, the wheel speed of the wheel on the high μ road surface side will not decrease below the wheel speed of the wheel on the low μ road surface side. Therefore, as long as the threshold value for starting pressure decrease control of the wheel on the low μ road surface side is set, the pressure decrease control for the wheel on the high μ road surface side also starts at the same time the pressure decrease control for the wheel on the low μ road surface side starts, so it is not particularly necessary to set the threshold value for starting the pressure decrease control of the wheel on the high μ road surface side.
Even with this control mode in which the select low control is employed, however, the braking force of the wheel on the high μ road surface side may become greater than the braking force of the wheel on the low μ road surface side due to variation in the hydraulic pressure control performance of the ABS control unit based on manufacturing error of a differential pressure control valve and depending on the effectiveness of the brakes. That is there is variation in the operational characteristics of the control valves used in ABS control which is caused, for example, by dimensional error of the structural members and variation in the constant of spring of elastic members. Even if the exciting current is the same, the control hydraulic pressure is not necessarily the same. In such cases, slip may be generated at the wheel on the high μ road surface side and the vehicle may become unstable.
This case will be described with reference to FIGS. 8A and 8B. FIGS. 8A and 8B are timing charts of the W/C pressure and the wheel speed, and the vehicle speed (estimated vehicle body speed) during braking on a split road surface when the select low control is employed. However, the W/C pressure and wheel speed shown in FIGS. 8A and 8B represent the W/C pressure and wheel speed of each wheel which is a wheel group in which ABS control is executed simultaneously for the left and right wheels. Further, the threshold value for starting pressure decrease control indicated in FIGS. 8A and 8B does not represent the slip ratio itself, but the wheel speed, with respect to the vehicle speed at that time, at which the threshold value for starting pressure decrease control will be reached.
As shown in FIGS. 8A and 8B, in the case of a split road surface, the way in which the wheel speed of the wheel on the low μ road surface side decreases with respect to the vehicle speed during braking is different from that of the wheel speed of the wheel on the high μ road surface side. That is, the wheel speed of the wheel on the low μ road surface side decreases more with respect to the vehicle speed than does the wheel speed of the wheel on the high μ road surface side.
At this time, if there is no variation in the braking force generated on the wheels of the high μ road surface side and the low μ road surface side, then the pressurization of the W/C pressure is equal, as shown in FIG. 8A. Therefore, while the wheel speed of the wheel on the high μ road surface side is substantially equal to the vehicle speed, the wheel speed of the wheel on the low μ road surface side exceeds the threshold value for starting pressure decrease control of the ABS control that was set for the wheel on the low μ road surface side and the pressure decrease control of the W/C pressure of the wheel on the low μ road surface side is started. At the same time, pressure decrease control of the W/C pressure of the wheel on the high μ road surface side is also started.
In such a case, stability of the vehicle can be maintained because there is virtually no decrease in the wheel speed of the wheel on the high μ road surface side with respect to the vehicle speed.
When there is variation in the braking force generated on the wheels on the high μ road surface side and the low μ road surface side, and the braking force of the wheel on the high μ road surface side is greater than that of the wheel on the low μ road surface side, the W/C pressure of the wheel on the high μ road surface side is pressurized sooner than the W/C pressure of the wheel on the low μ road surface side, as shown in FIG. 8B. Therefore, even if it takes only a short time at which the wheel speed of the wheel on the low μ road surface side decreases with respect to the vehicle speed and exceeds the threshold value for starting pressure decrease control of the ABS control, the wheel speed of the wheel on the high μ road surface side ends up decreasing with respect to the vehicle speed.
In such a case, the wheel on the high μ road surface side slips, causing a loss of vehicle stability.